As a result of the studies by DeLuca and Kodicek on the separation and identification of the metabolites of vitamin D.sub.3 and on its metabolism, it has been established that vitamin D.sub.3 is first hydroxylated (at 25-position) in the liver to form 25-hydroxyvitamin D.sub.3, then is hydroxylated (at 24R-position or at 1.alpha.-position) in the kidney to form 1.alpha.,25-dihydroxyvitamin D.sub.3 and 24R,25-dihydroxyvitamin D.sub.3 having hormone activity. It is also well known that these metabolites and other synthetic analogs such as 1.alpha.-hydroxy-vitamin D.sub.3 thereof enhance intestinal calcium transport and bone mineral mobilization and are useful as therapeutic agents to treat the diseases caused by various disorders in calcium metabolism.
A recent study based on experiments with a myeloid leukemia cell line (M1) isolated from an SL mouse with myeloid leukemia has revealed that the above named vitamin D.sub.3 derivatives can induce differentiation of myeloid leukemia cells and that they are at least about 100 times as potent as dexamethasone the most potent inducer ever known [Proc. Natl. Acad. Sci. U.S.A. 78, 4990 (1981)]. Sachs [Brit. J. Haematol., 40, 509 (1978)] and Hozumi ["Gan to Kagaku-ryoho (Cancer and Chemotherapy)", 8(1), 9, (1981)] have suggested that compounds capable of inducing differentiation of myeloid leukemia cells can be used to treat leukemia and that use of these compounds is promising as a supplement to the conventional chemotherapy and immunotherapy. But having a very great ability to induce differentiation, these derivatives also have significant effects on calcium metabolism in vivo, and an overdose of them may cause hypercalcemia. So, the derivatives are not completely satisfactory for use as a drug to treat leukemia which sometimes requires continuous administration of the drug in high dose.